This invention relates to a process for hydrotreating a heavy hydrocarbon oil containing asphaltenes and heavy metals in large amounts (hereinafter referred to as a "heavy oil") to produce a desulfurized hydrocarbon oil (hereinafter referred to as a "desulfurized oil").
Heavy oils including petroleum crude oils, residues obtained by distilling crude oil under atomospheric or reduced pressure and crude oils extracted from tar sands generally contain large amounts of so-called asphaltenes, heavy metal compounds, sulfur compounds, nitrogen compounds and the like. The sulfur and nitrogen compounds and heavy metals such as organo-metallic compounds of vanadium or nickel are contained in the heavy oils in extremely large quantities as contaminants and are concentrated in the fraction of high molecular hydrocarbons like asphaltenes and cause difficulties in the catalytic hydrodesulfurization of the oils. Because of these difficulties, the heavy oils which are present in nature in large amounts and which are regarded as a promising hydrocarbon resource for the future, are presently utilized only as low grade fuel oils or as asphalt for road paving. When used as a fuel oil, however, oxides of sulfur, nitrogen and heavy metals are discharged into the asmosphere as a result of their combustion and thus are unacceptable from an ecological standpoint.
In view of these considerations, techniques for converting heavy oils containing large amounts of asphaltenes into more valuable desulfurized and substantially asphaltene-free and heavy metal-free oils are being extensively investigated. The conventional techniques for obtaining desulfurized oil of high grade by the hydrodesulfurization of heavy oils include the so-called direct hydrodesulfurization and indirect hydrodesulfurization processes. The direct hydrodesulfurization process is carried out in a fixed bed or in an ebullated bed. The development of the direct hydrodesulfurization technique is indebted to improvements in catalyst performance. The development of the process is related to a determination of an important correlation between the properties of the raw material oil and the physical structure of the catalyst.
As will be fully understood by those skilled in the art of petroleum refining, several disadvantages result if asphaltenes and heavy metals are present in the raw material oil treated according to the direct hydrodesulfurization process. Thus, for example, since the asphaltenes colloidally dispersed in the raw material oil are huge molecules, it is difficult for them to diffuse to active sites within the pores of the catalyst. Because of this, the hydrodesulfurization is seriously inhibited. Moreover, the presence of asphaltenes accelerates the formation of coke and carbonaceous material resulting in a rapid lowering of the catalyst performance. Additionally, the heavy metals in the raw material oil accumulate on the surface of the catalyst and poison the catalyst considerably shortening the catalyst life.
One of the particularly important factors, therefore, for designing a catalyst for use in the direct hydrodesulfurization process is the choice of a catalyst having a pore size distribution which is adapted for the asphaltene content as well as the heavy metal content of the raw material oil. In accordance with the techniques of the prior art, when a raw material oil of comparatively good quality containing less than about 2 percent by weight of asphaltenes and less than 50 ppm of vanadium is subjected to hydrodesulfurization, a highly active hydrodesulfurization catalyst having pores with diameters as small as about 80 to 100 A is generally used. However, when a heavy oil of poor quality containing asphaltenes and vanadium in large amounts as high as about 2 to 5 percent by weight and 50 to 80 ppm, respectively, is subjected to hydrodesulfurization, not only does the resistance to diffusion within the pores become large, but the catalyst life is shortened markedly. Thus, it is substantially impossible to use the highly active catalyst having pores of only a small diameter. In order to increase the resistance of the catalyst to the poisoning of the catalyst by the asphaltenes and heavy metals and also to facilitate the diffusion to the active sites within the pores of the catalyst, even at the expense of sacrificing some of the catalyst performance, use is made of a catalyst having pores generally of a medium diameter of 100 to 150 A and, in some cases, even larger diameters. In some cases, however, a catalyst having an efficient activity can hardly be obtained so that an increased consumption of the catalyst results. Additionally, the operating conditions such as reaction temperature, liquid hourly space velocity (volume of reactor feed oil per volume of catalyst per hour hereinafter referred to as LHSV) must be controlled more severely. This causes further economically undesirable circumstances such as large hydrogen consumption, low yields of products, etc.
In the case of a heavy oil which is the subject of this invention and which contains more than 5 percent by weight of asphaltenes and more than 80 ppm of vanadium, it is considered to be a matter of course to employ a catalyst which is greatly resistant to poisoning having pores of a still larger diameter such as 200 A or more. The catalytic activity, however, of such a catalyst is too low and the reactivity of the sulfur compounds contained in heavy oils is also extremely low so that it is substantially difficult to hydrodesulfurize the heavy oil in accordance with one step by the prior art process for direct hydrodesulfurization.
In order to relieve the difficulties arising from the presence of asphaltenes and heavy metals in heavy oils a process has been proposed wherein a heavy oil is subjected to hydrotreatment to obtain a desulfurized oil by preliminarily hydrodemetallizing the heavy oil by the use of a comparatively inexpensive catalyst and then hydrodesulfurizing the resultant oil by the use of a hydrodesulfurization catalyst. This process is now being increasingly adopted. Problems still remain to be solved in this process, however, relating to the lowering of catalytic activity of the hydrodemetallization catalyst used in carrying out a continuous operation and relating to the regeneration or disposal of the spent catalyst and the like. Moreover, even though the catalyst poisoning due to heavy metals during hydrodesulfurization may be suppressed to some extent by the hydrodemetallization treatment, the problems relating to the poisoning and plugging of the desulfurization catalyst due to asphaltenes remain unsolved because the huge molecules of the asphaltenes in the heavy oils are essentially unchanged. On the other hand, in order to cause the cracking of the asphaltenes by the use of the conventional catalyst, extremely stringent reaction conditions should be employed. These reaction conditions, however, increase the hydrogen consumption as well as the catalyst consumption and thus the conventional process is industrially inpractical from the standpoint of economy.
In view of the present situation, therefore, a compromise has been adopted wherein the asphaltenes are preliminarily separated for removal by subjecting the raw material oil to physical treatment such as, for example, solvent deasphalting, and the resultant light fraction is subjected to hydrotreatment. Generally, the solvent deasphalting of the heavy oil is carried out using low molecular hydrocarbons such as propane, butane, pentane, etc. The asphaltene-containing fraction obtained as the byproduct can contain between 10 to 20 percent by weight, and in certain higher cases, 30% or more by weight of the raw material oil. Since the asphaltenes, as described above, contain concentrated amounts of contaminants such as heavy metals, sulfur compounds and nitrogen compounds, the resulting byproduct is extremely poor in quality and very low in utility and disposal of the byproduct is difficult. The conventional process, therefore, which includes the solvent deasphalting pretreatment is not economical and does not result in any significant improvement in the treatment of heavy oils containing asphaltenes in large quantities.
In the indirect hydrodesulfurization process the technical principle involved somewhat resembles that of the solvent deasphalting process. In the indirect hydrodesulfurization process the heavy oil is preliminarily subjected to vacuum distillation so as to separate a light fraction from a heavy fraction containing large amounts of asphaltenes and heavy metals. Only the light fraction which contains lower amounts of asphaltenes and heavy metals is hydrodesulfurized and then mixed with the above described heavy fraction. The liquid products obtained, however, contain asphaltenes as a matter of course so that the removal of the sulfur compounds, nitrogen compounds and heavy metals can be achieved only to a certain limited extent. As a result, the commercial value of the product is decreased.
It is an object of the present invention, therefore, to provide a process wherein heavy oils which contain asphaltenes in such large amounts that they can not be treated by the processes of the prior art are subjected to hydrotreatment in two efficient and consecutive steps to obtain desulfurized oil of low sulfur content. More particularly, it is an object of the first step of the process (step (a)) to subject a heavy oil to hydrodemetallization and simultaneous cracking of the asphaltenes by the use of a catalyst having a unique selectivity to produce a liquid product which is highly reactive for desulfurization and which is characterized by a molecular distribution within the range of 200 to 1200. It is an object of the second step of the process (step (b)) to subject the liquid products of the first step to hydrotreatment by the use of a hydrodesulfurization catalyst having a pore volume and pore distribution particularly adapted to convert the heavy oil to a desulfurized oil.
It is a further object according to the present invention to provide a hydroesulfurization of heavy oils containing extremely large amounts of asphaltenes and heavy metals in which catalyst consumption and hydrogen consumption are minimized by a process wherein hydrotreatment is carried out in two consecutive steps.